In the usual power transformer the coil structure comprises a low voltage coil surrounding a coil window and a high voltage coil surrounding the low voltage coil. The core structure in such a transformer comprises a core loop having a leg extending through the coil window. Should a short circuit occur on the load side of the transformer as a result of an equipment or cable failure, short-circuit currents, typically of high magnitude, will flow through the low voltage coil and produce high mechanical forces tending to collapse the low voltage coil onto the leg of the core loop.
In the traditional transformer having a core of silicon steel, the core leg has been able to resist such collapse of the surrounding low voltage coil without significant impairment. Such silicon steel core has sufficient stiffness and other properties to enable it to resist this mode of collapse without significant damage to the core. But in certain other types of transformers, the core does not have sufficient stiffness to resist such collapse of the surrounding coil without impairment. An example of such a transformer is one having a core of amorphous steel. Although amorphous steel cores have many advantages, they are subject to the disadvantage that they are sensitive to mechanical stresses, and core loss in such cores will significantly increase if they are subjected to the type of short-circuit produced stresses described hereinabove.